1. On remote SQL server, edit the /etc/mysql/my.cnf file and change the bind-address parameter in the mysqld section to make your server listening either on all interfaces (0.0.0.0) or a specfic IP address.
[mysqld] bind-address = 0.0.0.0
Once done, restart the MySQL server.
2. Always on the remote SQL server, create a specific SQL user with full privileges that can connect from the i-MSCP server only:
# mysql -u root -p<password> > GRANT ALL PRIVILEGES ON *.* TO '<username>'@'<imscp_server_ip>' IDENTIFIED BY '<password>' WITH GRANT OPTION; > quit;
Once done you must run the imscp-autoinstall script on the i-MSCP server and when you're asked for Mysql server, you must select the remote server option.